Toll-like receptors in the CNS: implications for neurodegeneration and repair.

The role of Toll-like receptors (TLRs) in the CNS is only starting to be uncovered. As in peripheral organs, multiple TLRs are dynamically expressed. They are involved in mounting a host-defense response against microbial invasion of the CNS. The many different TLRs expressed on microglia are likely the most important first line of defense in this respect. Intriguingly, microglial TLR tend to trigger a very standard cytokine and chemokine response, irrespective of the type of TLR agonist they meet. The main purpose of this standardized response by microglia may be to recruit the assistance by other cells rather than to immediately mount a destructive response toward invaders. As is generally the case for microglial responses, TLR-mediated responses can also work out in either beneficial or detrimental ways, depending on the strength and timing of the activating signal. Yet, the role of TLRs in the CNS extends well beyond controlling host-defense responses alone. Other cells in the CNS, including astrocytes, neurons, and oligodendrocytes, can also express multiple functional TLRs upon activation. These play important roles in tissue development, cellular migration, and differentiation; in limiting inflammation; and in mounting repair processes following trauma. The TLR-mediated reactions of these other neural cells to TLR agonists is highly cell specific and does not necessarily resemble that of microglia at all. It appears likely that endogenous agonists for TLRs are particularly relevant to activate these endogenous TLR functions on neural cells, also during development when microbial invaders have not yet entered the stage. In this chapter, current data are reviewed to highlight the emerging variety of functional roles of TLRs in the CNS.